This invention relates to a method and apparatus for separating particulates from a fluid using a centrifugal filtration device. More specifically, this invention relates to a method and apparatus for removing liquid or solid particles from a fluid, which can be a liquid or a gas, using a continuously cleanable high efficiency rotating filtration device utilizing microporous filtration material.
The removal of particulates from a fluid stream has long been a practice in a variety of industrial fields. Systems for filtering particulates from fluid streams include barrier and non-barrier inertial filtration devices. Barrier filtration devices can include porous media in the forms of filter bags, filter tubes, filter cartridges, and filter panels, while non-barrier devices can include electrostatic precipitators and inertial filtration devices such as rotating disk separators, cyclones, and venturi scrubbers.
Non-barrier, inertial devices offer several advantages over barrier filtration devices by avoiding reliance on barrier layers to stop and trap particles in fluids as the fluids pass through the layers. In one type of non-barrier, inertial device, the cyclone separator, contaminated fluid, such as a dust laden gas or particle-laden liquid tangentially enters near the top of an inverted cylindrical chamber. The curved tapering walls of a lower conical section of the cyclone impose a vortex motion on the incoming fluid, and the denser particles in the fluid are displaced towards the cyclone walls under the influence of centrifugal forces. The particles follow a downward spiral towards the cyclone exit near the apex of the cone, while separated fluid is drawn off from a central location in the upper portion of the cyclone. Cyclones are generally suitable for removing particles from gases when the particles are over about 5 xcexcm in diameter, and contain no filter elements that need periodic cleaning or replacement. Also, in multiple cyclone systems, 80% to 85% efficiency can be attained for removing particles of about 3 xcexcm diameter and above.
Another non-barrier, inertial device is a rotating disk separator, such as described in U.S. Pat. No. 2,569,567. Further refinement of rotating disk separators is embodied in U.S. Pat. No. 5,746,789. This type of separator comprises a plurality of annular disks rotatably mounted in a housing and concentrically stacked so that their hollow centers define a central plenum. Spacers closely regulate the gaps between the disks. One end of the plenum is sealed off with a cap, and the other end is left open and serves as a filtered fluid exit. As the disks rotate, particle-laden fluid enters the housing through an inlet and flows through the gaps between the rotating disks towards the plenum. Disk rotation creates boundary layers by imparting a rotational velocity component to particle laden fluid layers adjacent the disks, as the particle-laden fluid flows inwards and towards the plenum. The boundary layer fluid also imparts rotational velocity to particles entrained therein, which thereby experience a centrifugal force. Under appropriate conditions, the centrifugal force experienced by some particles can be greater than the drag forces on the particles caused by the fluid flow into the plenum. These particles are outwardly ejected from the rotating disk device. The fluid, now free of the ejected particles flows into the plenum and out the plenum exit. Rotating disk separators contain no filter elements that need periodic cleaning or replacement. Although these centrifugal filters have demonstrated submicron particle removal capabilities on a small scale, in practice, with higher volumetric flow rates common in many industrial applications, it is typically more challenging to remove particles smaller than about 2 xcexcm due to the higher rotational speeds and pressure drops required. Thus, particle filtration for fluids containing a high percentage of particles of less than about 2 xcexcm diameter using non-barrier, inertial devices remains impractical.
High efficiency barrier type filtration devices suitable for removing particles of less than about 2 xcexcm in diameter are known. In barrier layer devices, barrier layers comprise filtration media formed into filter elements through which the flow of particle-laden fluid is directed. Over time, filter element performance can deteriorate as filtered particles accumulate on the surface or through the depth of the filter element. The flow of fluids, whether liquid or gas, produces a pressure differential, or pressure drop, across the element. Preferably, the pressure differential is as small as possible for a given fluid flow rate in order to minimize the power required to filter the fluid. In many of these conventional techniques, filtration efficiency increases as filtered particulates accumulate on the filter element. However, as particles accumulate in or on the element, the pressure differential may increase, or the flow rate of fluid through the element may be reduced, or both. Therefore, after an amount of particulate material has accumulated or when limits of acceptable pressure differential or flow rate reduction have been reached, the filter element is either removed or cleaned.
Periodic element replacement and cleaning is generally needed to minimize filtration performance degradation in these systems. Filter element replacement and cleaning can be inconvenient and costly especially when considering the cost of shutting down industrial processes to allow this maintenance to be completed. In addition, the filter element can fail mechanically as a result of the stresses caused by cleaning the filter, thus resulting in loss of filtration performance.
Filter elements are typically constructed from filtration materials, or media such as, for example, felts and fabrics made from a variety of materials, including polyesters, polypropylenes, aramids, glasses and fluoropolymers. Selection of the type of material used is typically based on the fluid stream with which the filter element comes in contact, the operating conditions of the system and the type of particulate being filtered.
Polytetrafluoroethylene (PTFE) has demonstrated utility in many areas. As an industrial material, such as a filtration material, for example, PTFE has exhibited excellent utility in harsh chemical environments, which normally degrade many conventional metals and polymeric materials. A significant development in the area of particle filtration was achieved when expanded PTFE (xe2x80x9cePTFExe2x80x9d) membrane filtration media were incorporated as surface laminates on conventional filter elements. One example is taught in U.S. Pat. No. 4,878,930, directed to a filter cartridge for removing particles of dust from a stream of moving gas or air. Preferred filter materials for the cartridge are felt or fabric composites containing a layer of porous expanded polytetrafluoroethylene membrane.
Use of the ePTFE membrane greatly enhanced the performance of filter elements because the particles collected on the surface of the ePTFE, rather than in the depth of the elements as was occurring in the absence of the ePTFE layer. Several significant advantages were observed with these filter elements. For example, the filtration efficiency of the elements was high immediately from the outset of the filtration process, and it was not necessary to build up a cake of particles to achieve high efficiency.
Despite the superior performance and high filtration efficiency of cleanable ePTFE filtration media, filter element cleaning remains a problem, and filtration systems must frequently be shut down to remove and maintain filters, although cleaning methods have been developed to minimize these maintenance shut downs. For example, pulse jet cleaning, where the flow of the filtered fluid is temporarily reversed to dislodge accumulated material from the filter element surface, has been used for in situ filter element cleaning without shutting down the filtration system. However, in certain equipment where reverse fluid flow cannot be tolerated, back pulse or pulsejet cleaning cannot be used to clean the filter element.
Other methods of filtering, such as using rotating or centrifugal filters, have associated problems. Known centrifugal filters that use high efficiency filter media frequently do not work satisfactorily in single-stage filtration systems because they can quickly foul. High efficiency filter media are designed to remove high percentages of the particles in a fluid stream, where the particles are less than a specified size. Therefore, these filters are typically made from materials containing very small pores which are easily clogged and which can develop large capillary forces, trapping liquids that wick into the pores. In order to achieve acceptable fluid permeabilities in addition to high filtration efficiencies, filter media used in high efficiency filters are typically thin and weak, and are unable to withstand the centrifugal forces required to keep the media clean. Moreover, when laminated to backing materials for reinforcement, permeability can be reduced to unacceptable levels. When used upstream or downstream of other filtration systems, the expense and increased pressure losses of these rotating filters can offset the filtration performance benefits that may accrue from their use.
In addition, up to now, ePTFE filtration media have been found unsuitable for some applications. Despite high efficiency in some applications, ePTFE filtration media have been thought to be unsuitable for filtering gases containing entrained PTFE-wetting fluids. PTFE is hydrophobic and thus repels water. However, certain liquids, particularly oily liquids, tend to wet ePTFE surfaces. In addition, extended use can alter the surface energy of PTFE, and initially non-wetting fluids can eventually wet PTFE surfaces. During filtration, these wetting liquids can wick into the small pores of ePTFE filtration media and remain there, held by strong capillary forces. These capillary forces are inversely related to pore size. Thus, smaller pore sizes result in the generation of larger capillary forces. As filtration continues, wetting liquids blind ePTFE pores, resulting in reduced filtration media permeability and increased pressure drop across the filter. Even with thorough cleaning, a blinded filter element may not recover its full filtration performance.
In most industrial applications, non-barrier, inertial filtration devices alone cannot fully resolve these aforementioned problems economically. As discussed above, removing particulates of less than about 2 xcexcm diameter can be impractical using non-barrier, inertial devices that do not use porous filter media. In addition, barrier filtration devices containing microporous filtration media are susceptible to clogging and blinding. Thus, a need still exists for high efficiency continuously cleanable filtration devices.
The present invention is directed to a continuously cleanable, high performance filtration apparatus and a method for using the apparatus. The apparatus is a radial inflow centrifugal filtration device, which comprises a bulkhead abutting a chamber, the bulkhead having a filtered fluid outlet. The device further comprises a filter element located within the chamber and rotatably coupled at the filtered fluid outlet to the first bulkhead by a seal. The filter element of the present invention is generally tubular, its side-walls encircling and, thus, defining an interior plenum. The filter element side-walls incorporate filter media, most preferably laminated microporous membrane filter media.
Additionally, the radial inflow centrifugal filtration device can also include a counterflow boundary layer momentum transfer device. The boundary layer momentum transfer device of the present invention comprises a plurality of stacked annular disks having central openings, each disk separated from adjacent disks by a desired gap, wherein the central openings define a cavity in which the filter element is mounted.
The radial inflow centrifugal device can be used to filter fluid. A method of using the device includes flowing a particle-laden fluid from the chamber through the filter media in the filter element and out through the filtered fluid outlet. As the fluid flows through the device, the filter element is rotated at sufficient speed to eject particles from the filter media. The method also includes collecting particles from the particle-laden fluid on the filter media.